The invention discloses an assembling positioning device for a car oil filler cap and belongs to the field of motor vehicle assembling. The assembling positioning device comprises a first positioning body and a second positioning body fixedly connected with the first positioning body. The first positioning body is positioned on a side wall outer plate at a car oil filler and used for positioning the assembling positioning device in the left-right direction, the front-back direction and the up-down direction of a car. The second positioning body is connected with a car oil filler cap assembly and used for positioning the car oil filler cap assembly in the front-back direction of the car. The assembling positioning device for the car oil filler cap comprises a positioning system mounted on a car body and an oil filler cap assembling positioning system, so that the car oil filler cap assembly is accurately positioned on the car body, assembling is completed by fastening bolts at a time, and time and labor are saved; meanwhile, design of car body parts and design of the oil filler cap assembly do not need any change, and therefore the assembling positioning device can be applied to both developed cars in production and cars in development and design.